1. Field of the Invention
The present invention relates to a soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks, obtainable by forming a soluble lubricating film, and also relates to a method for manufacturing a fuel tank.
2. Description of the Related Art
The automobile fuel tank has a complicated shape in many cases and is required to have excellent processability (deep drawing property). Furthermore, the fuel tank is an important protective part of an automobile and therefore, as a matter of importance, the material used therefor must be a material which generates no corrosion product giving rise to the filter blocking, is free of any fear for formation of a corrosion hole and can be easily and stably welded. A Pbxe2x80x94Sn alloy plated steel sheet (see, Japanese Examined Patent Publication (Kokoku) No. 57-61833), which is a material having these various properties, is being widely used as the material for automobile fuel tanks. This material exhibits stable chemical properties against gasoline, excellent press shapability due to excellent lubricity brought by the plating, and good resistance welding property such as spot welding and seam welding. However, in recent years, an unleaded material is in demand in view of the load on the environment.
One fuel tank material containing no lead is an Al-base plated steel sheet. Al exhibits good corrosion resistance against gasoline, alcohol or organic acid resulting from deterioration of gasoline, because a stable oxide film is formed on the surface thereof. In producing a fuel tank using the Al plated steel sheet, the weldability and the processability are the problems. A treatment for improving these properties is proposed in Japanese Patent Application No. 8-287997.
On the other hand, a stainless steel sheet is a fuel tank material capable of satisfying the requirement for higher corrosion resistance demanded from the standpoint of eliminating the fear for a corrosion hole. The stainless steel sheet has both excellent corrosion resistance and good weldability and if it can be processed into a desired tank shape, the stainless steel sheet can be an excellent fuel tank material. However, the processability of the stainless steel sheet disadvantageously fails to cope with the complicated shape of fuel tanks in many cases.
The object of the present invention is to solve these problems and provide a soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks, which is obtainable by coating a soluble lubricating film on both surfaces or one surface. The object of the present invention includes providing a method for producing a fuel tank.
The present inventors have made extensive investigations to solve the above-described problems and obtain a soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks.
As a result, it has been found that the above-described objects can be attained by a stainless steel sheet obtained by coating a specific soluble lubricating resin film on both surfaces or one surface of an austenite-type stainless steel sheet, a ferrite-type stainless sheet or a two phase-type stainless steel sheet. The present invention has been accomplished based on this finding. More specifically, the soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the substrate stainless steel sheet is any one of an austenite-type stainless steel sheet, a ferrite-type stainless sheet and a two phase-type stainless steel sheet and on both surfaces or one surface thereof, a soluble lubricating resin film is formed.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the substrate stainless steel sheet is any one of an austenite-type stainless steel sheet, a ferrite-type stainless sheet and a two phase-type stainless steel sheet and on both surfaces or one surface thereof, a soluble lubricating resin film mainly comprising (A) a soluble polyurethane resin composition containing a carboxyl group or a sulfonic acid group within the molecule and having a glass transition point of 100xc2x0 C. or more as a dry film and (B) a lubricating function-imparting agent in an amount of 1 to 30% by mass based on the soluble polyurethane resin composition is formed. By coating with this soluble lubricating resin film, the obtained soluble lubricating surface-treated stainless steel sheet for fuel tanks can provide excellent shapability even under severe conditions in press molding such as deep drawing and ironing.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the substrate stainless steel sheet is any one of an austenite-type stainless steel sheet, a ferrite-type stainless steel sheet and a two phase-type stainless steel sheet and on both surfaces or one surface thereof, a soluble lubricating resin film mainly comprising (A) a soluble polyurethane resin composition containing a carboxyl group or a sulfonic acid group within the molecule and having a glass transition point of 100xc2x0 C. or more as a dry film, (B) a lubricating function-imparting agent in an amount of from 1 to 30% by mass based on said soluble polyurethane resin composition and (C) silica particles in an amount of 1 to 30% by mass based on said soluble polyurethane resin composition is provided. By adding silica particles within the above-described range, the adhesive property between the soluble lubricating resin film and the stainless steel surface is improved and furthermore, the alkali-soluble lubricating film is improved in the film strength and in the galling resistance.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the amount of an acid group contained in the soluble polyurethane resin composition constituting the soluble lubricating film is from 30 to 180 in terms of an acid value. By introducing a compound having in the polyurethane molecule an acid group in an amount of 30 to 180 as an acid value, the coating can have adhesive property to stainless steel and thereby follow the molding even under severe molding conditions despite the fact that the glass transition temperature of the dry film is 100xc2x0 C. or more.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the neutralizer for the acid group contained in (A) the soluble polyurethane resin composition for forming a soluble lubricating resin film is sodium hydroxide or potassium hydroxide. Since the neutralizing agent for acid group is sodium hydroxide or potassium hydroxide, the solubility necessary in the present invention can be achieved.
The term xe2x80x9csoluble lubricating resin compositionxe2x80x9d means that the composition can be formed into an aqueous solution, emulsion or dispersion to coat the composition on a stainless sheet and the composition after it is coated and dried can have a lubricating property and can be solved with an alkali degreasing solution or hot water, for example, so that the composition can be removed from the surface of the stainless sheet.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the main component constituting (A) the soluble polyurethane aqueous composition for forming a soluble lubricating resin film is polyester polyol. Since the main component constituting (A) the soluble polyurethane aqueous composition is polyester polyol, the solubility necessary in the present invention can be achieved even at low temperatures such as room temperature.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the main component constituting (A) the soluble polyurethane aqueous composition for forming a soluble lubricating resin film is polyether polyol. Since the main component constituting (A) the soluble polyurethane aqueous composition is polyether polyol, the coating can have flexibility capable of following the molding even under severe molding conditions despite the fact that the glass transition temperature of the dry film is 100xc2x0 C. or more.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the lubricating function-imparting agent (B) for forming the soluble lubricating film comprises one or more members selected from the group consisting of polyolefin-based wax, fluorine-containing wax, paraffin-based wax and stearic acid-based wax. By the addition of this lubricating function-imparting agent, a low coefficient of dynamic friction is ensured over a wide temperature range and good lubricating performance can be obtained.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the substrate is an austenite-type stainless steel sheet comprising, in % by mass, C: 0.5% or less, Si: 5% or less, Mn: 15% or less, P: 0.1% or less, S: 0.05% or less, Ni: 6 to 20%, Cr: 15 to 30%, N: 0.5% or less, and Al: 0.001 to 5% with the balance consisting of Fe and inevitable impurities. On considering the performance as the tank material, such as corrosion resistance and processability, and the production cost, an austenite-type stainless steel sheet comprising the components satisfying the above-described range is preferred.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the substrate is a ferrite-type stainless steel sheet comprising, in % by mass, C: 0.5% or less, Si: 3% or less, Mn: 5% or less, P: 0.1% or less, S: 0.05% or less, Ni: 5% or less, Cr: 9 to 30%, N: 0.2% or less, and Al: 0.001 to 5% with the balance consisting of Fe and inevitable impurities. On considering the performance as the tank material, such as corrosion resistance and processability, and the production cost, a ferrite-type stainless steel sheet comprising the components satisfying the above-described range is preferred.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks is characterized in that the substrate is a two phase-type stainless steel sheet comprising, in % by mass, C: 0.5% or less, Si: 5% or less, Mn: 15% or less, P: 0.1% or less, S: 0.05% or less, Ni: 2 to 20%, Cr: 12 to 30%, N: 0.5% or less, and Al: 0.001 to 5% with the balance consisting of Fe and inevitable impurities. On considering the performance as the tank material, such as corrosion resistance and processability, and the production cost, a two phase-type stainless steel sheet comprising the components satisfying the above-described range is preferred.
The soluble lubricating surface-treated stainless steel sheet with excellent shapability for fuel tanks according to the present invention is characterized in that the substrate stainless steel sheet, which is any one of austenite-type stainless steel sheet, ferrite-type stainless steel sheet and two phase-type stainless steel sheet, further contains one or more of, in % by mass, Mo: 0.01 to 8%, Cu: 0.01 to 5%, Ti: 0.01 to 1%, Nb: 0.01 to 1%, V: 0.01 to 1%, Mg: 0.001 to 0.1%, Ca: 0.001 to 0.1%, B: 0.0005 to 0.05% and W: 0.01 to 5%.